Solution and method for brightening
cadmium



Re. 26,130 Reissued Dec. 27, 1966 26,130 SOLUTION AND METHOD FORBRIGHTENING CADMlUM Kenneth P. Bellinger, Ellington, and Eugene G.Cllapdelaine, Hazardville, Conn., assignors to Conversion ChemicalCorporation, Rockville, Conn., a corporation of Connecticut No Drawing.Original No. 3,072,516, dated Jan. 8, 1963, Ser. No. 97,164, Mar. 21,1961. Application for reissue Jan. 9, 1964, Ser. No. 348,311

7 Claims. (Cl. 156-20) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

The present invention relates to a novel solution and method for thechemical brightening of cadmium surfaces.

The term cadmium surfaces as used herein refers to the surfaces ofcadmium and cadmium base alloy articles and other metallic articleshaving a coating of cadmium such as produced by electroplating.

It is an aim of the present invention to provide a method for thebrightening of cadmium surfaces which is rapid and relativelynon-critical so as to enable greater uniformity of operation andappearance.

It is also an aim of the present invention to provide a method forbrightening cadmium surfaces by a simple and inexpensive one-steptreatment which further provides mild corrosion and tarnish protectionfor the cadmium surface.

Another aim is to provide a method for brightening cadmium surfaceswherein the rinsing of the workpiece subsequent to the action of thebrightening bath is relatively non-critical so as to provide attractive,uniformly brightened workpieces with minimal rejection.

A further aim is to provide an easily transported and handled dry powdercomposition for addition to an aqueous acid bath in relatively smallamounts for the production of a dilute cadmium brightening solution toattain the foregoing aims.

Other advantages will be readily apparent from the following detailedspecification and claims.

It has now been found that the foregoing and related aims can beobtained by a method in which the cadmium workpieces are immersed in anacid bath consisting essentially of dilute aqueous solution of trivalentchromium ions, nitrate ions, [and ions selected from the groupconsisting of fluoride, sulfate, and combinations thereof] and thecombination of fluoride and sulfate ions. The pH of the bath should bemaintained between about 1.0-3.5, and preferably in the range of 1.5-2.0for short dips of up to 10 seconds, and in the range of 1.8-3.5 for dipsup to one minute or longer.

Hexavalent chromium ions have been found to produce a dulling effectupon the cadmium surfaces so that the presence of hexavalent chromium isto be avoided. In baths having a pH of about 1.0-2.0, a concentration ofas little as 0.02 gram per liter of hexavalent chromium constitutes theupper limit of tolerance, and a dulling surface effect is noticeable.With baths of higher pH on the order of 2.0-3.5, a greater concentrationof hexavalent chromium can be tolerated, although the amount is stillrelatively small and less than 0.3 gram per liter. The bath itself isnon-oxidizing with respect to the trivalent chromium so that onlyimpurities in the components or impurities carried into the bath uponthe workpieces need be considered. The baths of the present inventionare thus essentially characterized as substantially free from hexavalentchromium ions.

Generally, the bath should contain trivalent chromium ions in the rangeof 008-10 gram per liter, nitrate ions in the range of about 0.1l8.0grams per liter, [and ions selected from the group consisting offluoride, sulfate] and the combination [thereof,] of fluoride andsulfate ions in the range of 0.4-5.2 grams per liter.

The [preferred] solutions in accordance with the present inventioncontain both sulfate and fluoride ions since solutions containing bothions have been found to provide the cadmium surface with an appreciabledegree of protection to corrosion by fume and moisture. Generally, thefluoride ion concentration in such solutions should be about one-thirdthat of the sulfate ion and be in the range of about 0.1-1.2 grams perliter, and the sulfate ion concentration about 0.25-4.0 grams per liter.

The initial presence of zinc and/or cadmium ions in the bath has alsobeen found desirable in controlling the action of the bath upon thecadmium surface and in providing uniform brightness. These metallic ionsare preferably added as the sulfate salt so as to provide the desired pHby the nitric acid and limit the introduction of any unessential ions.Generally, when such ions are to be incorporated, they should bepresented in the amount of about 0.1-3.0 grams per liter.

The bath of the present invention is preferably utilized at ambienttemperatures on the order of 6090 F. for optimum operation. However,elevated temperatures of -150 F. may be employed depending upon theconcentration of the bath and the time of treatment. Generally, the moredilute the bath, the higher the temperature that may be employed so asto provide a satisfactory treatment time and brightening action.

The time of treatment will vary with the temperature, pH andconcentration of the bath. Generally, the period of immersion will varybetween about 2 seconds and 10 minutes depending upon these threefactors. The optimum time of treatment for a given bath concentration,temperature and pH is best determined by visual observation of aspecimen workpiece. Generally, the preferred baths of the presentinvention operated at ambient temperatures require an immersion periodof only about 5-45 seconds.

Subsequent to immersion in the bath of the present invention, thecadmium workpieces are rinsed in cold water and then rinsed in hot waterfor drying.

The preferred baths of the present invention are those containing about0.2-0.3 gram per liter of trivalent chromium ions, 3.75-5.0 grams perliter of nitrate ions, 0.2- 0.4 gram per liter of fluoride ions and0.75-1.5 grams per liter of sulfate ions, the bath having a pH of about1.5-2.0 and being substantially free from hexavalent chromium ions.Additionally, 0.5-1.0 gram per liter of zinc or cadmium ions isdesirable for control and optimum brightening action during initial use.Such a bath composition produces a highly desirable luster and mildprotection upon the cadmium surface at ambient temperatures in 5-45seconds immersion.

Generally, the bath is prepared by addition to water of nitric acid andsuitable salts of the various other ions. As can readily be appreciated.the introduction of unnecessary ions is desirably avoided sinceinterference may be provided thereby. Thus, it has been found that theuse of a dry powder composition containing chromium fluoride, sodiumacid fluoride, and zinc or cadmium sulfate has been economical for theconsumer while providing highly desirable results and enabling easyshipment. Complex fluoride salts can also be employed such as thefluoroborates and fluosilicates.

A specific dry powder composition which is readily transported andutilized for the production of cadmium brightening baths of the presentinvention is one containing the following salts in the indicatedproportions:

Percent by weight Sodium acid fluoride 6-8 Chromium fluoride 20-22 Zincor cadmium sulfate 70-72 Total product 100 This composition is then bestutilized by an admixture of 1-4 pounds of the above, 100 gallons ofwater and 0.1-4.0 quarts of 42 Baum nitric acid, and preferably 1.0-3.0quarts of nitric acid for a bath requiring a relatively short immersiontime of about to 15 seconds.

Because of the acidity and corrosive nature of the baths of the presentinvention, suitable tanks should be employed to avoid contamination suchas rigid polyvinyl chloride, steel lined with polyvinyl chloride orresinbonded glass fibers or polyethylene.

From time to time, the bath will have to be replenished by the additionof the several components as the concentration is reduced by carry-outand reaction. This can generally be determined by visual observation oflesser brightening of the workpieces. Generally, addition of one-halfthe original makeup amounts is conveniently employed when such visualreduction is observed. Analytically, the bath can be titrated with 1 Nsodium hydroxide using phenolphthalein as an indicator. For ex ample,when using the formulation set forth with respect to the dry powdercomposition, the following test is conveniently employed. For a 100 cc.sample, 10 cc. sodium hydroxide is required for optimum bathconcentration. For every cubic centimeter of sodium hydroxide less than10 cc.. 3V4 ounces of the dry powder composition and 6 /1 fluid ouncesof 42 Baum nitric acid are added per 100 gallons of bath. Of course, thepH should be maintained within the described limits during operation.

Indicative of the eflicacy of the present invention are the followingspecific examples wherein cadmium-plated workpieces were treated.

EXAMPLE 1 A cadmium brightening bath was prepared by adding to water 42Baum nitric acid and a dry powder mix containing 7.12 percent by weightsodium acid fluoride, 21.25 percent by weight chromium fluoride, and71.63 percent by weight zinc sulfate. The nitric acid and dry powder mixwere added in a ratio of two quarts and two pounds, respectively, per100 gallons of water. The resulting bath had a pH of about 1.8 and wasmaintained at a temperature of about 75 F.

Test panels of steel plated with cadmium to a thickness of about 0.0002inch and measuring 1 /2 inches by 11/4 inches were immersed in the bathfor ten seconds, rinsed in cold water for three seconds, rinsed in hotwater (180 F.) for three seconds and air dried. The treated panels wereobserved to have a lustrous finish of uniform brightness and wereobserved to be much brighter than an untreated control panel specimen.

To measure the protection afforded by the treatment, specimens weresubjected to a humidity test in which they were exposed to 100 percenthumidity at a temperature of 100 F. for forty-eight hours. At the end ofthis exposure, there was no evidence of spotting out" or formation ofdark blemishes, which is indicative of the mild protection afforded bythe treatment of the present invention.

4 EXAMPLE 2 A bath was prepared in accordance with Example 1 andmaintained at a temperature of about F. Test specimens similar to thoseemployed in Example 1 were immersed in the bath for about eight seconds,rinsed in cold water, rinsed in hot water and air dried. The resultantpanels had a uniform luster much brighter than an untreated controlpanel.

One of the tests utilized in the industry to evaluate protection is thatof placing a drop of 1.0 percent by weight lead acetate solution on thetreated part and determining the time in which the spot turns black. Anuntreated specimen will turn black immediately. When the treated panelswere tested, the spot turned grey in about five to ten seconds andcontinued darkening until completely black at the end of about fifty tosixty seconds. The treatment is thus seen to have provided mildprotection for the cadmium surface in addition to high brightness.

We claim:

1. An acid bath for the brightening of cadmium surfaces comprising adilute aqueous solution containing essentially 0.08-1.0 gram per literof trivalent chromium ions, 0.1-l8.0 grams per liter of nitrate ions and0.4-5.2 grams per liter of the combination of fluoride and sulfate ions[selected from the group consisting of fluoride, sulfate and thecombination thereof], said solution having a pH of about 1.0-3.5 andcontaining at least 0.1 gram per liter of fluoride ion and 0.25 gram perliter of sulfate ion, said solution containing less than about 0.3 gramper liter of hexavalent chromium ions.

2. The bath in accordance with claim 1 containing 0.25-4.0 grams perliter of sulfate ion and 0.11.2 grams per liter of fluoride ion, withthe fluoride ion being about one-third the concentration of the sulfateion.

3. An acid bath for the brightening of cadmium surfaces and providing amild protection therefor comprising a dilute aqueous solution containingessentially 0.2-0.3 gram per liter of trivalent chromium ions, 3.75-5.0grams per liter of nitrate ions, 0.2-0.4 gram per liter of fluorideions, 0.75-1.5 grains per liter of sulfate ions and 0.5-1.0 gram perliter of ions selected from the group consisting of zinc, cadmium andthe combination thereof, said solution having a pH of about 1.5-2.0 andcontaining less than about 0.02 gram per liter of hexavalent chromiumions.

4. As a dry powder additive for dilute solutions of nitric acid toprovide an acid bath for the brightening of cadmium surfaces, :1 mixturecontaining essentially about 6-8 percent by weight of sodium acidfluoride, 20-22 percent by weight chromium fluoride, and 70-72 percentby weight of the sulfate salt of a metal selected from the groupconsisting of zinc and cadmium.

5. The method of brightening cadmium surfaces comprising providing adilute aqueous containing essentially 0.08-1.0 gram per liter oftrivalent chromium ions, 0.1- 18.0 grams per liter of nitrate [ions and0.4-5.2 grams per liter of ions selected from the group consisting offluoride, sulfate,] and the combination [thereof] of fluoride andsulfate ions, said solution having a pH of about 1.0-3.5 and containingat least 0.1 gram per liter of fluoride ion and 0.25 gram per liter ofsulfate ion, said solution containing less than about 0.3 gram per literof hexavalent chromium ions; maintaining said solution at a temperatureof about 45-150 F.; immersing a cadmium workpiece in said solution for aperiod of time sufficient to brighten the surface thereof; and rinsingsaid workpiece in cold water.

6. The method of brightening cadmium surfaces comprising providing adilute aqueous solution containing essentially 0.2-0.3 gram per liter oftrivalent chromium ions, 3.75-5.0 grams per liter of nitrate ions,0.1-0.4 gram per liter of fluoride ions, 0.751.5 grams per liter ofsulfate ions and 0.5-1.0 gram per liter of ions selected from the groupconsisting zinc, cadmium, and the combination thereof, said solutionhaving a pH of about 1.5-2.0 and containing less than about 0.02 gramper liter of hexavalent chromium ions; maintaining said solution atambient temperature; immersing a cadmium workpiece in said solution fora period of about 5-45 seconds sufficient to brighten the surfacethereof; and rinsing said workpiece in cold water.

7. The method of claim 5 wherein said bath contains 0.25-41) grams perliter of sulfate ion and 0.1-1.2 grams per liter of fluoride ion.

References Cited by the Examiner The following references, cited by theExaminer, are of record in the patented file of this patent or theoriginal patent.

UNITED 6 STATES PATENTS Wilhelm 148-62 Dupernell et 211.

Johnson 148-6.21 Hesch 1562l Hampel.

Ostrander et al.

Ncwell et al.

Bellinger et a1. l5620 Canada.

ALEXANDER WYMAN, Primary Examiner.

15 JACOB H. STEINBERG, Examiner.

